This invention relates to methods of manufacture of brazed bodies, and abrasive articles made thereby.
Brazing is a process of joining or uniting an assembly of two or more materials into one structure. Brazing is achieved by heating the materials to a temperature below the solidus temperature of both the materials in the presence of a braze and an optional flux. A brazed body is a metal to metal bond joined by braze. In some instances, the flux is added to reduce the surface of one or both of the metals prior to the braze, creating a surface more agreeable to wetting and therefore achieving a better bond.
Some known fluxes, such as those containing non-metallic ionic salts such as borides and fluorides, work by dissolving oxides at low temperatures. For example, a fluoride flux commercially available under the trade name HANDY FLUX (commercially available from Lucas Milhaupt, Inc., Cudahy, Wis.), contains fluorides that begin to dissolve oxides at 315xc2x0 C. and remain active from 590xc2x0 C. to 870xc2x0 C. These fluxes can cause undesirable results in some circumstances, because this type of flux can leave residual flux and reaction products of the flux within the brazed body that eventually contribute to corrosion spots in the brazed body. Additionally, these fluxes may deplete the surface of a metal of non-corrodible materials, leaving only corrodible materials in its place, for example leaving an iron-rich surface on stainless steel. Minimizing the amount of this type of flux assists in reducing the amount of corrosion that may eventually appear in a brazed body.
A preferred environment for brazing is a vacuum furnace because it minimizes the oxygen in the atmosphere available to attack the hot metals. Fluxes containing borides and fluorides are too volatile to be used in a vacuum furnace, and even a vacuum furnace is unable to stop all metals from oxidizing on their surface. An example of such a metal is stainless steel. An oxide of chromium on the surface of stainless steel inhibits the wetting of stainless steel parts, even in a vacuum furnace. One known solution is to plate a thin layer of nickel over the stainless steel. The braze will wet the nickel surface and metal diffusion will increase the bond between the plated nickel and the base of stainless steel. While plating the stainless steel with nickel works, it requires an expensive plating step and introduces quality assurance problems in making sure the plated nickel meets the requirements for brazing such as uniform coverage and adhesion.
Another known solution is mixing hydrogen into the vacuum furnace to react with any oxygen in the furnace, thereby delivering an oxygen-free environment as described in Brazing of Stainless Steel, Paul F. Stratton, Heat Treating Progress, p.p. H14-H16 (August 2000). However, in actual practice, an oxygen free environment is nearly impossible to achieve. This is especially true for certain metals (e.g. chromium) that oxidize so readily that they would require an extremely dry hydrogen atmosphere.
Therefore, it is desired to have a brazing flux that is non-volatile in the vacuum furnace and still reduces the surface of metals, especially easily oxidized metals. Additionally, it is desirable to have a simple and inexpensive method of brazing metals that are difficult to join.
In one embodiment of the invention, the invention comprises a method of manufacturing a brazed body. The method comprises forming a multi-layer assembly comprising: a first material capable of forming a first oxide and having a melting temperature higher than 660xc2x0 C.; a first reducing metal adjacent the first material, the reducing metal capable of reducing at least a portion of the first oxide on the first material a braze adjacent to the reducing metal; and a second material adjacent the braze, the second material comprising a material having a melting temperature higher than 660xc2x0 C. The method then comprises creating a vacuum around the assembly, and heating the assembly to melt the reducing metal and the braze. The assembly is then subject to cooling to thereby form the brazed body.
Another aspect of the invention provides a brazed body comprising a first layer comprising a first material, the first material having a melting temperature higher than 660xc2x0 C.; a second layer comprising a second material, the second material having a melting temperature higher than 660xc2x0 C.; and a filler layer between the first layer and the second layer, the filler layer comprising a multi-phase alloy having a braze and aluminum oxide.
The invention may also provide an abrasive article comprising a first layer comprising a fused abrasive body; a second layer comprising a material having a melting temperature higher than 660xc2x0 C.; and a filler layer between the first layer and the second layer, the filler layer comprising a multi-phase alloy comprised of a braze and a reducing metal oxide. Another embodiment of the invention comprises an abrasive article comprising a plurality of abrasive particles; a metal having a melting temperature greater than 660xc2x0 C.; and a filler layer between the metal and the abrasive particles comprising braze and aluminum oxide. Additionally, the invention provides for a multi-layer brazing assembly comprising a braze metal foil; and an aluminum coating covering at least part of one surface of the braze, the aluminum coating being at least 8 micrometers thick.
As used herein, the following terms have the following definitions:
xe2x80x9cFluxxe2x80x9d refers to a material that removes oxides from a surface of a base metal and promotes the wetting of the base metal.
xe2x80x9cNative oxidexe2x80x9d refers to a metal oxide formed from a metal without any additional oxide coatings.
xe2x80x9cReducing Metalxe2x80x9d refers to a metal that forms an oxide having a free energy of formation that is less than the free energy of formation of the oxide of another metal it is being introduced to reduce.